- Adds a new S3 class (cnd_df) for represented conditioned data frames, i.e. data frames that carry metadata about what records should be used for derivations

- Adds support for basic pretty printing of cnd_df objects

- Adds a user-facing function for creating such cnd_df objects: `condition_by`

- Adds experimental "mutate"-version function for these conditioned data frames: `derive_by_condition()`

- Joins by raw and target data sets are now aware of conditioned tibbles
- Transformation functions, namely `assign_datetime()`, `hardcode*()` and `assign*` are also conditioned-tibble aware
- Unit test coverage for most cases indicated at #54

I believe the essential components are here to support the if_then_else algorithm via conditioned tibbles. Now, further testing, assertions and documentation is needed.

… help with template code

Merge branch '0054-condition-by' into 51-documentation-updates-v01
# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

- Move `tgt_dat` to the first position in the argument list for cleaner command pipes.

- Rename `condition_by()` to `condition_add()`.

- Export `oak_id_vars()` for direct user access.

- Update tidyselections to align with the latest practices.

Merge branch '0054-condition-by' into 51-documentation-updates-v01
# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

Merge branch 'main' of github.com:pharmaverse/sdtm.oak into 51-v2

# Conflicts:
#	NAMESPACE
#	inst/WORDLIST
#	renv/profiles/4.4/renv.lock